linux-kernel - Re: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall

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Date:   Thu, 24 Sep 2020 09:16:48 -0500
From:   YiFei Zhu <zhuyifei1999@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "containers@...ts.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dimitrios Skarlatos <dskarlat@...cmu.edu>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Hubertus Franke <frankeh@...ibm.com>,
        Jack Chen <jianyan2@...inois.edu>,
        Jann Horn <jannh@...gle.com>,
        Josep Torrellas <torrella@...inois.edu>,
        Kees Cook <keescook@...omium.org>,
        Tianyin Xu <tyxu@...inois.edu>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Tycho Andersen <tycho@...ho.pizza>,
        Valentin Rothberg <vrothber@...hat.com>,
        Will Drewry <wad@...omium.org>
Subject: Re: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall_arches[] array

On Thu, Sep 24, 2020 at 8:47 AM David Laight <David.Laight@...lab.com> wrote:
> I doubt the compiler will do what you want.
> Looking at it, in most cases there are one or two entries.
> I think only MIPS has three.

It does ;) GCC 10.2.0:

$ objdump -d kernel/seccomp.o | less
[...]
0000000000001520 <__seccomp_filter>:
[...]
    1587:       41 8b 54 24 04          mov    0x4(%r12),%edx
    158c:       b9 08 01 00 00          mov    $0x108,%ecx
    1591:       81 fa 3e 00 00 c0       cmp    $0xc000003e,%edx
    1597:       75 2e                   jne    15c7 <__seccomp_filter+0xa7>
[...]
    15c7:       81 fa 03 00 00 40       cmp    $0x40000003,%edx
    15cd:       b9 40 01 00 00          mov    $0x140,%ecx
    15d2:       74 c5                   je     1599 <__seccomp_filter+0x79>
    15d4:       0f 0b                   ud2
[...]
0000000000001cb0 <seccomp_cache_prepare>:
[...]
    1cc4:       41 b9 3e 00 00 c0       mov    $0xc000003e,%r9d
[...]
    1dba:       41 b9 03 00 00 40       mov    $0x40000003,%r9d
[...]
0000000000002e30 <proc_pid_seccomp_cache>:
[...]
    2e72:       ba 3e 00 00 c0          mov    $0xc000003e,%edx
[...]
    2eb5:       ba 03 00 00 40          mov    $0x40000003,%edx

Granted, I have CC_OPTIMIZE_FOR_PERFORMANCE rather than
CC_OPTIMIZE_FOR_SIZE, but this patch itself is trying to sacrifice
some of the memory for speed.

YiFei Zhu